This invention relates to coated compositions of water-soluble polymers, their manufacture and their use in enhanced oil recovery.
The world energy crisis has made it essential to recover as much oil as possible from oil-fields.
One of the most widely used methods of enhanced recovery consists of flooding the oil-field with injected saline water, thus obliging the oil to cool off and leave the pores of the rock where it stands absorbed.
The efficiency of this method is however often impaired by the difference of viscosity between oil and water, which difference causes water to flow through preferential paths directly from the injection well into the production well, instead of spreading through the whole deposit.
It is usual to reduce this viscosity difference by thickening the injection water with water-soluble polymers, very often with a polyacrylamide, optionally partially hydrolysed, or with a polysaccharide.
The storage and handling of these polymers on the field has raised a number of practical problems.
As a matter of fact, the powdered polymers have the property to absorb water from the air. This absorption results in swelling and formation of agglomerates and/or microgels.
The agglomerates tend to stick on the walls and thus to inhibit the running of the mixing devices, particularly the feed screw.
The microgels do not dissolve easily in water and, once injected into the oil-field, they tend to clog the openings of the pores of the oil-containing rock.
On the other hand, the polymer powder spreads in the air and on the ground, which raises safety problems by making the ground thick and slippery and the atmosphere hardly breathable.
To obviate these drawbacks, the prior art has proposed a number of processes to disperse the solid particles of water-soluble polymers in an organic liquid which is a non-solvent for the polymer.
For example, fluid dispersions of water-soluble polymers have been obtained by adding the polymers to an anhydrous non-solvent medium, in the presence of a surface-active agent and optionally a thickening agent to make the suspensions stable; such compositions have been described in the U.S. Pat. No. 3,282,874, the British Pat. No. 1,397,933 and in the European patent application No. 002 368 and they are characterized by the use of a hydrocarbon which is liquid at room temperature, as a non-solvent medium for the polymers.
The prior art has also taught to coat the water-soluble polymers, so as to protect them, either with sorbitol or mannitol, as disclosed, for example, in the Japanese patent application No. 4 744 335, or with a monoalkylether of diethylene glycol, as disclosed, for example, in the U.S. Pat. No. 3,350,338. However, taking the sensitivity of these coating agents to water into account, the protection of the polymers is not completely satisfactory.
It is thus desirable to obtain stabilized compositions in the solid state, in order to improve and simplify the handling and the storing of the water-soluble polymers, and this is the object of this invention.